Correction circuits



Jan. 7, 1941. E. w. HEROLD I 2,223,081

' CORRECTION CIRCUITS Filed Aug. 24, 19:59

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Patented Jan. 7, 1941 PATENT OFFECE 2,228,081 CORRECTION CIRCUITS Edward W. Herold, Verona, N. .L, assignor to Radio Corporation of America, a corporation of Delaware Application August 24,

10 Claims.

My present invention relates to correction circuits generally, and more particularly to signal transmission tube circuits utilizing specially designed networks for correcting various undesired conditions.

One of the main objects of my invention is to provide a novel means whereby undesirable effects caused by the finite time of transit of the electrons in certain types of vacuum tube are cancelled.

Another important object of this invention is to provide a means whereby a tube whose control grid lies in a current-limited region may be used in an amplifier or converter whose input is tunable over a wide frequency range without the limitation of amplification by electronic transittime effects.

Another object of this invention is to provide a tube amplifier or converter whose performance is made substantially independent of transit time effects by the connection of an external impedance, and which is capable of operating up to frequenciesof 100. megacycles (mc.) and higher.

Yet another object of my invention is to provide a frequency converter arrangement for superheterodyne receivers in which the undesired coupling between oscillator and signal circuits, and which is largely due to electronic effects within a tube, are cancelled over a wider fre- 30 quency range than has heretofore been possible.

Still other objects of my invention areto-improve generally the stability andefiiciency of high frequency amplifiers and converters, and more especially to attain these ends in av simple 35 and economical manner.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims; the invention itself, however, as toboth its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing:

Fig. 1 shows one form of the present invention applied to a tunable high frequency amplifier.

Fig. 2 shows curves of the electronic input conductance of a particular tube when used alone, and. when. combined with two forms of. the present invention, and is included to show the behaviorofthe invention in a specific example, and

Fig. 3 shows the present invention applied to 1939, Serial No. 291,630

the. cancellation of electronic coupling effects in a converter tube.

Among the chief limitations of amplifier and converter tubes to be operated at high frequencies are electronic effects which are caused by the 5 finite time of transit of electrons in their passage through the tube. With the conventional spacecharge control tube, for example, it is found that the conductive component of the input impedance increases with frequency to a point where 10 it may be the limiting factor in the amplification of the tube. One method of improving such tubes for use at high frequencies has been to decrease the inter-electrode spacings so as to reduce the electronic transit time. Another im- 15 provement may be effected by operation at higher voltage. Both these methods, however, have serious structural imitations and cannot be said to have extended the practical upper frequency limit of operation of tubes to more than three or 20 four times that formerly attained. Furthermore, below the limiting frequency at which amplification is no longer possible, the operation of spacecharge control tubes is accompanied by considerable input loading so that the latter becomes an extremely important factor in circuit design.

An entirely different solution to the problem is presented by my invention which makes use of tubes in which space-charge control is not the o predominant factor whereby the anode current of a tube is regulated. In the type of tube known as a space-charge grid tube, for example, the electrons approaching the control electrode do so at high velocity and are decelerated in approaching it. The control electrode in such a tube does not necessarily operate by varying the spacecharge conditions adjacent to it, but may be made to operate merely as a gate which allows more or fewer electrons to passdependir ggn its potential. The effect of electronic transit time on the input impedance of the control electrode may be quite different in such tubes over the effect in the more conventional space-charge controlled tubes wherein electrons leave a thermionic cathode at very low velocities and are eventually accelerated towards the adjacent control grid. In fact, it has been found that the input conductance of a suitably-biased control grid in a spacecharge grid tube is negative rather than positive as with the conventional tube. While the positive input conductance of the conventional tube reduces the amplification by its loading of the input circuit, the negative input conductance of a properly operated space-charge grid tube actually increases the amplification.

It has been found that a negatively-biased electrode exhibits negative conductance when it is so operated as to be in a current-limited region. With the conventional tube, for example, if the cathode temperature is reduced to the point at which the entire emission current leaves it, the control grid conductance is found to be negative. Under such conditions, however, the control grid potential would aiTect the current only slightly, and the tube would be virtually useless as an amplifier. The control electrode of a properly constructed and operated space-charge grid tube, however, may be made to have a very great control action, and yet have a negative conductance. It has also been found that the negative input conductance increases approximately in magnitude with the square of the frequency as: gin=kw where gm is the input conductance, w is the angular frequency, and 7c is a constant which depends on the tube configuration and operating voltages. At low frequencies, therefore, as with most other transit-time efiects, the input conductance is frequently so small that it may be neglected. At high frequencies, however, the

conductance becomes important and may be sufficiently negative to excite oscillations in the input circuit.

An amplifier or converter tube whose input conductance varies markedly with frequency is not always desirable, and is particularly objectionable in amplifiers Whose input circuits must be tuned over a wide range. When the input conductance is positive, it is objectionable because the amplification is reduced by varying amounts depending on the frequency, while, when it is negative, the amplification is increased by varying amounts and instability may result due to self-oscillation at some adjustments.

It is the purpose of my invention to reduce the variable negative conductance substantially to zero at all frequencies by the utilization of an equal and opposite positive conductance which varies in the same way.

Considering a circuit consisting of a resistance R and capacitance C in series, it is found that such a circuit behaves similarly to a parallel circuit of a conductance Geff and capacitance Ceff where: i

and w is the angular frequency. For frequencies below a certain point It is evident that, if Ic=C R, the total con- G total against frequency.

ductance will be substantially zero, independently of frequency up .to some limiting point By making C larger and R smaller, the upper frequency limit may be extended as much as desired. It is, of course, undesirable to add an excessive amount of capacitance, and it is, therefore, impracticable to use such a C and R circuit for correction up to too high a frequency for a given tube.

Considering a circuit consisting of inductance L, resistance R and capacitance C in series and with L: C'R I have found that such a combination behaves similarly to a parallel circuit of conductance Gm and capacitance Ceff where 1 +gw C R l +-la*C R This series circuit behaves substantially as a positive conductance increasing as the square of the frequency up to a much higher frequency than the simple C and R series circuit. For a 10% departure from the square-law relation, the L, C, R circuit is satisfactory up to Where the simple C and R circuit is satisfactory only to Thus, the L, C and R circuit may be connected to a tube with a control electrode in a currentlimited region to obtain a zero input conductance amplifier or converter up to a much higher frequency for the same added capacitance, or up to the same frequency for a much reduced capacitance as compared with the O-R circuit. It is, of course, necessary that the conditions be substantially realized in order to achieve this favorable result.

Fig. 1 shows a form of the invention as applied to a high frequency amplifier tunable over a wide range. An amplifier tube I comprising a cathode 2, a control electrode 3, an anode 4 and auxiliary electrodes 5 and 6 is shown connected to a signal input circuit 7, and an output circuit 8. A positive potential is applied to the auxiliary electrodes 5 and 6 which serve as a space-charge grid and screen grid respectively. By-pass condensers 9 and ID are included to insure that alternating potentials are not present on electrodes 5 and 6. The control electrode 3 is operated in a substantially current-limited region between electrodes 5 and 6, and has an input conductance whose value is negative and increases as the square of the frequency following the relation: gm=kw A series circuit of capacitance l I, resistance 12 and induct-ance I3 is connected between the control electrode and ground, and is so chosen that L= C'R and C R=k.

. Fig. 2 shows the manner in which the total conductance caused by the tube varies with frequency with and without the connection of the compensating circuits. The curve labelled No correction shows the measured input conductance of a particular amplifier tube as plotted Examination of the data shows that this tube approximately obeys the relation gm=0.5 10- w Making and choosing a small value for C such as 4 mmf., the values C=4 mmf., R2311 ohms are obtained. The curve labelled CR correction shows the input impedance withv this C and R series circuit acrossthe tube. The input conductance will be seen to be approximately zero up to nearly 30 megacyc'les. By adding a series inductance L: CR =0.2 x 10- henries, the curve labelled CR-L correction is obtained. This curve gives an approximately Zero input conductance up to nearly 100 megacycl-es. It should be noted that inasmuch as the value of inductance required in series with the C-R combination is quitesmall, it may frequently be obtained through the use of lead wires to C and R of the proper length and geometrical configuration.

Another undesirable electronic effect which is found in some types of converter tube at high frequencies may also be corrected according to my invention. Fig. 3 shows the invention as applied to the reduction of space-charge coupling in a pen tagrid converter. The converter tube I I has the grids l2, I3, I, I5, 16 interposed in the order named between cathode l1 and anode l8. Signal voltages are applied to the tunable input circuit I9, the high potential end of which is connected to the signal grid I 5. Bias for the latter may be obtained from either the usual self-bias network in the cathode lead, or from an A. V. C. network. The oscillator section of the converter comprises cathode l1, oscillator grid I2, oscillator anode-grid l3 and the circuits associated therewith. The tunable circuit I3 connected between oscillator grid l2 and cathode H determines the frequency of the local oscillations. The intermediate frequency output circuit 20 is connected in circuit with plate I8. The inter-mediate frequency oscillations are utilized in any manner well known to those skilled in the art.

As has been explained in my U. S. Patent 2,141,750 granted December 27, 1938, the opera tion of a pentagrid converter is accompanied by an electronic space-charge coupling between oscillator and signal circuits which may be neutralized or cancelled by the connection of a suitable network. In the aforementioned patent it was shown that a series connection of a condenser and resistance was effective to secure the space charge coupling neutralization over a fairly wide range of frequencies up to an upper limit frequency of about 40 me. It is not believed necessary to repeat here the theoretical aspects of the neutralization action, as the aforesaid patent fully explains the design and effects of the CR neutralization network.

In the present arrangement there is employed a series path comprising capacity 2|, resistor 22 and inductance 23 as the neutralization network. Cancellation of the space charge coupling effect between the oscillator and signal grids is secured thereby over a wider frequency range than with the series C-R circuit of the aforesaid patent. For best results the same values of C and R (21 and 22) are chosen as in the circuit of my said patent. The value of inductance 23 is made approximately equal to The space charge coupling isthereby effectively cancelled to a frequency approximately twice as high as with inductance 23 omitted. Since the value of L23 is usually quite small (just as in the correction of input conductance as hereinbefore explained), it may frequently be obtained through the use of lead wires to C and R of the proper length and geometrical configuration. Although the circuit shown in Fig. 3 is for a pentagrid converter it is obvious that the method applies to any mixer or converter tube in which innergrid oscillator injection is used.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What I claim is:

1. In a high frequency amplifier, an electron discharge tube of the type whose control grid is surrounded by a positive potential field and is in a current-limited region and has a negative input conductance, a signal input circuit coupled to the control grid, and means electrically associated with the input circuit for cancelling the effect of the negative input conductance of said tube.

2. In a. high frequency amplifier, an electron discharge tube of the type whose control grid is surrounded by a positive potential field and is in a current-limited region and has a negative input conductance, a signal input circuit coupled to the control grid, and means comprising capacitance and resistance electrically associated with the input circuit for cancelling the effect of the negative input conductance of said tube.

3. In a high frequency amplifier, an electron discharge tube of the type whose control grid is surrounded by a positive potential field and is in a current-limited region and has a negative input conductance, a signal input circuit coupled to the control grid, and means electrically associated with the input circuit for cancelling the effect of the negative input conductance of said tube, said means consisting of a series path of capacitance and resistance connected across said input circuit.

4. In a high frequency amplifier, an electron discharge tube of the type whose control grid is surrounded by a positive potential field and is in a current-limited region and has a negative input conductance, a signal input circuit coupled to the control grid, and means electrically associated with the input circuit for cancelling the effect of the negative input conductance of said tube, said means consisting of capacitance, resistance and inductance arranged in series across the input circuit.

5. In a high frequency amplifier, an electron discharge tube of the type whose control grid is surrounded by a positive potential field and is in a current-limited region and has a negative input conductance, a signal input circuit coupled to the control grid, and means electrically associated with the input circuit for cancelling the effect of the negative input conductance of said tube, said means consisting of resistance, capacitance and inductance chosen to cancel said negative conductance over a range up to 100 megacycles.

6. In an amplifier network of the type utilizing a tube whose control grid is surrounded by a positive potential field and is in a current-limited region and whose input conductance is negative, and a circuit connected to said control grid for cancelling the effect of said negative conductance over a wide range of frequencies.

'7. In an amplifier network of the type utilizing a tube whose control grid is surrounded by a positive potential field and is in a current-limited region and whose input conductance is negative, and a circuit, comprising at least capacitance and resistantce arranged in series, connected to said control grid for cancelling the effect of said negative conductance over a wide range of frequencies.

8. A frequency mixing network comprising a multi-electrode tube, a signal frequency circuit connected to one of the electrodes, a local oscillator circuit connected to another of the electrodes, said last two electrodes having undesired interaction, and means for cancelling said interaction comprising capacitance, resistance and inductance connected in series between said two electrodes, and said inductance being approximately equal to one half the product of the capacitance and the square of the resistance.

9. In a high frequency amplifier, an electron discharge tube of the type whose control grid is surrounded by a positive potential field and is in a current-limitedregion and has a negative input conductance, a signal input circuit coupled to the control grid, and means electrically associated with the input circuit for cancelling the efiect of the negative input conductance of said tube, said means consisting of capacitance, resistance and inductance arranged in series across the input circuit, and said inductance being approximately equal to one-half the product of the capacitance and the square of the resistance.

10. In a high frequency signal-translating circuit which includes an electron discharge device having at least two electrodes between which undesired phenomena are present, said undesired phenomena exhibiting the effect of a substantially constant negative capacitance and a negative conductance whose value increases with frequency, a correcting circuit comprising a positive capacitance, a positive resistance and a positive inductance connected in series between the two aforesaid electrodes and proportioned so as to substantially cancel at least the said negative conductance effects over a wide frequency range and said inductance having a magnitude equal to one half the product of the capacitance and the square of the resistance.

EDWARD W. HEROLD. 

